Respiratory flow sensor

ABSTRACT

A respiratory tract differential-pressure flow sensor, including a tube having a distal end that is insertable into a respiratory tract, an interfering body disposed within the tube proximal to the distal end and having a first edge facing one end of the tube and a second edge facing away from the end, the interfering body extending across the diameter of the tube, a first pressure sensing port operative to sense an air pressure, the first port being disposed in the first edge not abutting the wall of the tube, and a second pressure sensing port operative to sense an air pressure, the second port being disposed in the second edge not abutting the wall of the tube, where either of the edges is inclined inwardly towards the axis of the interfering body extending across the diameter of the tube.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 10/200,764, filed Jul. 24, 2002, entitled “Respiratory FlowSensor,” and incorporated herein by reference in its entirety.

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates generally to the field of medical devicesand specifically to medical devices that are designed to monitor therespiratory characteristics of patient breathing, especially for thosepatients who are attached to mechanical ventilators.

Persons exhibiting acute or chronic respiratory failure, for example dueto pulmonary infection or trauma, often require artificial ventilatorysupport and may therefore be connected, by means of flexible plasticventilation tubing, to a mechanical ventilator. Correct functioning ofsuch a ventilator entails continuous, accurate and reliable monitoring,by the ventilator, of airflow characteristics within the connectingplastic tubing. Such monitoring is often achieved by means of a gas-flowsensor interposed within the plastic ventilation tubing connecting thepatient to the ventilator.

It will be well known to those familiar with the art that manymechanical ventilators utilize a flow sensor fashioned in the form of abore of cylindrical tubing containing within it a strut, also known asan interfering body, in a manner which facilitates differential pressuremeasurements, at either end of the strut, that are proportional to theflow rate of the respiratory gases that pass through the sensor. Such aflow sensor is hereinafter referred to as a differential-pressure flowsensor, and is described in more detail below.

A differential-pressure flow sensor typically is comprised of a hollowcylindrical body having a bore, which can be connected between aventilator and a patient. The differential-pressure flow sensor utilizesan aerodynamic strut that is disposed within the cylindrical bore of thesensor to create a drop in the pressure of the respiratory gases flowingthrough the sensor. The strut extends across the entire diameter of thebore of the flow sensor and bisects the circular bore of the sensor. Thewidth of the strut is less than the diameter of the bore and thelongitudinal length of the strut is less than the length of the bore.Further, the geometric cross section of the strut is symmetrical to theflow of respiratory gases flowing through the sensor in eitherdirection, and has a generally elliptical cross section. The aerodynamicdesign of the strut preserves the laminar nature of airflow through thecylindrical bore as the air passes around the strut, such that airflowturbulence is minimal or absent within the flow sensor.

The aerodynamic strut has longitudinally exposed edge portions, suchthat when the differential-pressure flow sensor is interposed between apatient and a ventilator one edge portion is closer to the patient andthe other edge portion is closer to the ventilator. Each of the edgeportions of the aerodynamic strut contains a semicircular groove runningthe full height of the edge, that is, parallel to the short axis of thecylindrical body and extending from the inner surface of the cylindricalbody on one side to the inner surface of the cylindrical body on theopposite side of the cylindrical body. One end of each groove is incontinuity with a circular lumen within the wall of the cylindricalbody. This circular lumen is thus located at the intersection betweenthe inner surface of the cylindrical body and the edge portion of theaerodynamic strut. The circular lumen extends through the wall of thecylindrical body. On the outer surface of the cylindrical body thislumen receives tubing, which runs from the differential-pressure flowsensor to a pressure transducer, typically located within the mechanicalventilator.

The differential pressure measured by the flow sensor is due to therestriction to flow caused by the presence of the strut within the boreof the sensor. The drop in pressure is measured relatively between thegroove in the first edge portion of the aerodynamic strut and the groovein the second edge portion of the aerodynamic strut. For example, whenrespiratory gases are flowing through the flow sensor from the first endto the second end, a high-pressure zone, also referred to as an area ofstatic pressure, is created immediately adjacent to the first edge ofthe strut and a low-pressure zone, also referred to as an area ofdynamic pressure, is created immediately adjacent to the second edge ofthe strut. The converse is true when the respiratory gases are flowingfrom the second end of the sensor toward the first end of the sensor. Itshould be emphasized that in terms of the functioning of thedifferential-pressure flow sensor, the actual point at which pressure ismeasured, using the venturi principle, is at the circular lumen on theinner surface of the cylindrical body, and that this pressuremeasurement reflects the pressure along the length of the groove in theedge portion of the aerodynamic strut.

The relative pressures of the respiratory gases flowing through thesensor are collected and conveyed to the pressure transducer through thecircular lumens and the tubing connected thereto. The pressuretransducer measures the received pressures, and the resultant data isthen processed by a microprocessor so as to calculate the rate of gasflow through the differential-pressure flow sensor. This calculation isbased on the principle that the drop in pressure across an obstructionin an airway is related to the square of the velocity of the fluidsflowing through the airway. This principle is also true for thedifferential-pressure flow sensor. The general relationship between theflow velocity and the pressure drop as measured across the strut by thetransducer is given by:(flow velocity)² ΔP

-   -   where ΔP is the drop in pressure across the strut of the        differential-pressure flow sensor. This relationship is unique        for every unique flow sensor geometry and must be derived        empirically. Accordingly, the plastic flow sensors that are used        with a given ventilator and microprocessor are manufactured from        the same molds and injection conditions so that the geometric        variation between each flow sensor is negligible.

Determining the flow-to-pressure drop relationship is accomplished byforcing air through the differential-pressure flow sensor atpredetermined flow rates and measuring the resulting changes indifferential pressure across the strut through the lumens, so as togenerate a set of data points. A high order linear equation is then fitto the data points. This equation closely follows the same general formas given above. Using this equation, a flow velocity for gases flowingthrough the differential-pressure flow sensor can be calculated from thedifferential pressures measured across the strut.

It is known, however, that standard differential-pressure flow sensors,as described above, suffer from several deficiencies. In particular,standard differential pressure flow sensors have been found to functionunreliably in the presence of high humidity within the respiratorygasses flowing through the sensor. Humidification of the inspiredrespiratory gasses is often achieved by flowing the respiratory gassesthrough a water humidifier before the gasses pass through the flowsensor and into the patient. This is desirable so as to prevent dryingof the patient's respiratory tract mucousa during prolonged periods ofmechanical ventilation. Humidity may also be introduced into therespiratory gasses in the form of aerosolized medications, which arefrequently administered to mechanically ventilated patients. Evenwithout the introduction of external humidity, the naturally expiredrespiratory gasses from a patient's lungs are of higher humidity thanthe inspired gasses, thus increasing the humidity of airflow through thedifferential-pressure flow sensor.

As the humidity of the respiratory gasses increases, water condensationmay occur on the inner surface of the respiratory tubing and thedifferential-pressure flow sensor. When such condensation occurs in thecircular lumen in the inner wall of the flow sensor, at which sitepressure measurements are sensed, water blocks the lumen, thusdistorting the pressure measurements recorded by thedifferential-pressure flow sensor, and invalidating the resultant flowdata. Furthermore, water that has previously condensed elsewhere alongthe length of the flexible respiratory tubing may flow into the flowsensor due to movement of the tubing, and cover the pressure sensinglumen. The propensity for the pressure sensing lumen to become blockedby water is exacerbated by the narrow gauge of the lumen and itsconnected tubing, which causes liquid to enter the tubing by means ofcapillary action.

Several different solutions have been developed in an attempt toovercome this deficiency of differential-pressure flow sensors. Onealternative has been to insert a heating electrode into the flow sensorso as to heat the inner surface of the sensor and thereby prevent watercondensation. This technique requires the addition of electrical wiringand machinery to the flow sensor and ventilator, thus increasing thecost and mechanical complexity of the sensor. A second alternative hasbeen to try ensure that the sensor remains oriented in space in such away that the pressure-sensing lumen is on the superior aspect of thecylindrical body, rather than the dependent aspect where condensed waterwill accumulate due to gravity. This alternative has proven to beimpractical, as movement of the patient or the flexible respiratorytubing inevitably results in movement of the flow sensor, and thusmovement of the accumulated water within the sensor, causing blockage ofthe pressure-sensing lumen.

There is therefore a need for, and it would be highly advantageous tohave, a differential-pressure flow sensor that prevents condensation ofwater vapor in the pressure-sensing lumens of the sensor, and thatprevents blockage of the pressure-sensing lumens by condensed waterwhich may flow into the sensor from the respiratory tubing. It would bedesirable for such a sensor to achieve these aims without the additionof electrical components to the sensor.

SUMMARY OF THE INVENTION

The invention is a differential-pressure gas flow sensor, for use inmechanical ventilators, wherein the shape of the aerodynamic strut, orinterfering body, prevents the lumens at which the differentialpressures are sensed from becoming obstructed by condensed water. Threeunique characteristics of the shape of the interfering body, whichprevent water blockage of the pressure-sensing lumen from occurring andwhich are points of novelty of the current invention, are:

1) For each edge portion, the pressure-sensing lumen is located-on theedge portion of the interfering body-distant from the inner surface, andcloser to the central axis of symmetry, of the cylindrical body.Consequently, accumulation of condensed water in the dependant part ofthe sensor (along its inner surface) does not block the pressure-sensinglumen, which remains above the water level.

2) The walls of the interfering body slope convergently from theelliptical base of the interfering body towards its center, rather thanbeing essentially parallel to each other. This unique shape of theinterfering body results in airflow patterns around the interfering bodywhich generate turbulent boundary layer flow patterns near the base ofthe interfering body, distant from the pressure-sensing lumens. Asturbulent boundary layers encourage water condensation andprecipitation, these processes occur primarily at the base of theinterfering body, rather than at the pressure-sensing lumens.

3) Each edge portion of the interfering body slopes at an angle from thebase of the interfering body to the location of the pressure-sensinglumen. Consequently, the leading edge of the interfering body (namely,the leading edge which faces towards the source of airflow) deflects theairflow along a flow vector oriented towards the pressure-sensing lumenon the opposite (trailing) edge portion in such a way as to flushcondensed water out of the area of the pressure-sensing lumen.

In one acpect of the invention a differential-pressure flow sensor isprovided, including a) an interfering body, having a first edge,disposed within a tube, the interfering body extending across thediameter of the tube, and b) a first pressure sensing port operative tosense an air pressure, the first port being disposed in the first edgenot abutting the wall of the tube.

In another aspect of the present invention the first edge is inclined atan angle with respect to the axis of the interfering body extendingacross the diameter of the tube.

In another aspect of the present invention the angle is about 13degrees.

In another aspect of the present invention the first edge is a leadingedge with regard to a direction of airflow.

In another aspect of the present invention sides of the interfering bodyare convergent with each other along the diameter of the tube.

In another aspect of the present invention sides of the interfering bodyconverge at an angle of about ten degrees with respect to the axis ofthe interfering body extending across the diameter of the tube.

In another aspect of the present invention sides of the interfering bodyare concave.

In another aspect of the present invention the pressure sensing port isdisposed upon the first edge.

In another aspect of the present invention the pressure sensing port isrecessed within the first edge.

In another aspect of the present invention the pressure sensing port hasa diameter of about 1.54 millimeters.

In another aspect of the present invention the pressure sensing port isdisposed in the first edge at about the midpoint of the first edge.

In another aspect of the present invention the tube is operative to holda volume of liquid, and where the port is disposed in the first edge ata distance from a wall of the tube that is greater than a depth of thevolume of liquid.

In another aspect of the present invention the interfering body isoperative to disturb an airflow, the disturbed airflow including aboundary layer in proximity to the wall of the tube.

In another aspect of the present invention the boundary layer isturbulent.

In another aspect of the present invention the tube is Y-shaped.

In another aspect of the present invention the tube is respiratorytubing.

In another aspect of the present invention the tube is an endotrachealtube.

In another aspect of the present invention the interfering body has asecond edge, and further includes c) a second pressure sensing portoperative to sense an air pressure, the second port being disposed inthe second edge not abutting the wall of the tube.

In another aspect of the present invention the second edge is a trailingedge with regard to a direction of airflow.

In another aspect of the present invention the first edge is operativeto deflect an airflow along a vector directed towards the secondpressure sensing port.

In another aspect of the present invention a method for measuringairflow is provided, including a) providing a differential-pressure flowsensor, the sensor including an edge and a pressure-sensing port, and b)deflecting an airflow from the edge along a vector directed towards thepressure-sensing port sufficient to remove liquid from thepressure-sensing port.

In another aspect of the present invention a method for measuringairflow through a tube is provided, including a) providing adifferential-pressure flow sensor, the sensor including an interferingbody, the interfering body being operative to disturb airflow throughthe tube, and b) disturbing the airflow through the tube around theinterfering body, the disturbed airflow including a turbulent boundarylayer in proximity to the wall of the tube.

In another aspect of the present invention a method for measuringairflow through a tube is provided, including a) providing adifferential-pressure flow sensor, the sensor being operative to sense apressure, and b) sensing a pressure at a location not abutting the wallof the tube.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, withreference to the accompanying drawings, wherein:

FIG. 1 is an illustration of the overall structure of the flow sensor;

FIG. 2 is a longitudinal sectional side view of the flow sensor;

FIG. 3 is a longitudinal sectional top view of the flow sensor;

FIG. 4 is a short-axis view of the flow sensor;

FIG. 5 is an illustration of airflow patterns caused by the leading edgeof the interfering body;

FIG. 6 is an illustration of a preferred embodiment of the flow sensor;

FIG. 7 is an illustration of an alternative embodiment of the flowsensor; and

FIG. 8 is an illustration of an alternative embodiment of the flowsensor.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is a differential-pressure respiratory flow sensorfor use in a mechanical ventilator.

The principles and operation of a differential-pressure respiratory flowsensor, according to the present invention, may be better understoodwith reference to the drawings and the accompanying description.

FIG. 1 shows the overall structure of the current invention. The figurecan best be understood by simultaneously referring to FIGS. 2, 3, and 4,which show side, top and short-axis views respectively of the devicedepicted in FIG. 1. A flow sensor 2 comprises a hollow cylindrical body4 having a bore, and an interfering body 6 disposed within the bore ofcylindrical body 4. In the preferred embodiment, cylindrical body 4 is asegment of respiratory tubing, although it will be understood that anyform of tubing may be used without departing from the spirit of thecurrent invention. In terms of the current invention, the word “tube” ishereby defined as referring to a hollow conduit of any cross-sectionalshape, including circular, elliptical and square.

Interfering body 6 extends across the entire diameter of the bore ofcylindrical body 4 and bisects the circular bore of cylindrical body 4.The width of interfering body 6 is less than the diameter of the bore ofcylindrical body 4 and the longitudinal length of interfering body 6 isless than the length of the bore of cylindrical body 4. As can be seenin FIG. 3, the geometric cross section of the strut is symmetrical tothe flow of respiratory gases flowing through sensor 2 in eitherdirection, and when represented in two dimensions approximates a rhombusor ellipse in shape, as will be described below. As such, two corners ofinterfering body 6, at either end along its length, constitutelongitudinally exposed edge portions 8 and 10 when represented in threedimensions. When airflow occurs through sensor 2, edge portions 8 and 10are thus leading and trailing edges, depending on the direction ofairflow. In addition, the two corners of interfering body 6 at eitherend along its width constitute lateral edges 102 and 104.

As can be seen in FIG. 2, interfering body 6 is wider superiorly, whereit is in contact with the superior aspect of cylindrical body 4, thanwhat it is inferiorly, where it is in contact with the inferior aspectof cylindrical body 4. Hereinafter, the wider, superior end ofinterfering body 6 will be referred to as the “base” of interfering body6, and the narrower, inferior end will be referred to as the “inferiorinsertion” of interfering body 6.

A midpoint 32 bisects edge portion 8 into a first length 20, runningfrom the base of interfering body 6 to midpoint 32, and a second length36, running from midpoint 32 to the inferior insertion of interferingbody 6. With regard to FIGS. 2 and 4, the term “horizontal” will referto a plane that is parallel to the long axis of cylindrical body 4 andthe term “vertical” will refer to a plane that is perpendicular to thelong axis of cylindrical body 4. In a preferred embodiment, midpoint 32is generally located at the mid-point of the vertical length of edgeportion 8, that is, on the central axis of symmetry of cylindrical body4. In an alternative embodiment, midpoint 32 may be locatedasymmetrically along the length of edge portion 8, that is, closer toone side of cylindrical body 4 than to the opposite side of cylindricalbody 4, at essentially any location on edge portion 8. It is aparticular feature of sensor 2 that first length 20 tapers at an inclinefrom the inner surface of cylindrical body 4 towards midpoint 32. In apreferred embodiment, first length 20 is inclined at an angle of 13degrees from the vertical plane, although other degrees of angulationmay be used without departing from the spirit of the current invention.Second length 36, in contrast to first length 20, is orientedvertically, and is recessed into the body of interfering body 6 relativeto first length 20. A semicircular groove 12 runs along the length ofsecond length 36. A horizontal shelf 40, at the same vertical locationon edge portion 8 as midpoint 32, lies between the medial end of firstlength 20 and semicircular groove 12. Horizontal shelf 40 contains acircular lumen 16 adjacent to semicircular groove 12. In a preferredembodiment, the diameter of circular lumen 16 is 1.54 mm. A bore 24 runsfrom circular lumen 16, within the body of interfering body 6 and inproximity to first length 20, to the outer surface of cylindrical body4. At the outer surface of cylindrical body 4, bore 24 receives tubing28. Tubing 28 runs from flow sensor 2 to a pressure transducer (notshown).

As can be seen in FIG. 4, that part of lateral edge 102 which extendsfrom the base of interfering body 6 to the vertical level of circularlumen 16 (hereinafter referred to as the “upper part” of lateral edge102) tapers at an incline. In a preferred embodiment, this inclinationis at an angle of 10 degrees from the vertical plane, although otherdegrees of angulation may be used without departing from the spirit ofthe current invention. In contrast, those parts of lateral edge 102 thatextends from the vertical level of circular lumen 16 to the inferiorinsertion of interfering body 6 is oriented vertically. The dimensionsand structure of lateral edge 104 are identical, in a mirror image, tothose of lateral edge 102.

Edge portion 10 and its surrounding surfaces are essentially a mirrorimage of edge portion 8 and its surrounding surfaces. Thus a midpoint 34bisects edge portion 10 into a first length 22 and a second length 38.First length 22 tapers at an incline (as described for edge portion 8)from the inner surface of cylindrical body 4 towards midpoint 34. Secondlength 38 is oriented vertically, and is recessed into the body ofinterfering body 6 relative to first length 22. A semicircular groove 14runs along the length of second length 38. A horizontal shelf 42 liesbetween the medial end of first length 22 and semicircular groove 14.Horizontal shelf 42 contains a circular lumen 18 adjacent tosemicircular groove 14. A bore 26 runs from circular lumen 18, withinthe body of interfering body 6 and in proximity to first length 22, tothe outer surface of cylindrical body 4. At the outer surface ofcylindrical body 4, bore 26 receives tubing 30. Tubing 30 runs from flowsensor 2 to a pressure transducer (not shown).

In a preferred embodiment of the current invention, the upper parts oflateral edges 102 and 104 (as depicted in FIG. 4) and first lengths 20and 22 of edge portions 8 and 10 (as depicted in FIGS. 1 and 2) arestraight. It is a particular feature of this embodiment, however, thatin the upper part of interfering body 6 (that is, the part between thebase of interfering body 6 and the vertical level of circular lumens 16and 18), the external surfaces of interfering body 6 that lie betweenlateral edges 102 and 104 and edge portions 8 and 10, are not straight,but are concave in shape. Thus, the horizontal, geometric cross sectionof interfering body 6 changes, depending on the vertical level at whichthe cross section is depicted. As can be seen in FIG. 3, the horizontalcross-section of interfering body 6 at its base is essentiallyelliptical (indicated as contour “A” in FIG. 3), whereas the horizontalcross-section of interfering body 6 at the vertical level of circularlumens 16 and 18 approximates a rhombus with rounded corners (indicatedas contour “B” in FIG. 3). In a preferred embodiment, ellipse “A” has acircumference of 47.9 mm, a length of 17.7 mm, and a width of 6 mm,while contour “B” can be described as follows: the four corners of therhombus-like shape are arcs of 3 mm diameter circles, and are connectedto each other by tangents of those circles. The two circles whose arcspass through edge portions 8 and 10 have a distance of 12 mm betweentheir centers, and the two circles whose arcs pass through lateral edges102 and 104 have a distance of 4 mm between their centers. The concaveslopes of interfering body 6 have the effect of optimizing airflowpatterns around interfering body 6 so as to prevent airflow turbulencefrom occurring in the flowing gas outside of the boundary layers. Thisconfiguration is particularly effective when cylindrical body 4 isfashioned in a Y-type configuration, as described below with regard toFIG. 6.

The upper part of interfering body 6 can thus be described as being asegment of an elliptically-based cone. This conical aspect ofinterfering body 6, whereby the sides of interfering body 6 slopeinwards from the base of interfering body 6, is a novel feature of thecurrent invention. When air flows through sensor 2, the inward slopingof the walls of interfering body 6 generates a pattern of airflow whichcharacteristically includes a turbulent boundary layer around the baseof interfering body 6. As will be well known to one familiar with theart, the interfering body of existing differential-pressure airflowsensors is designed so as to facilitate laminar airflow through thesensor, and specifically avoid turbulence. As such, the sides of theinterfering body are essentially parallel to each other, a designfeature that is aerodynamically advantageous for the purpose ofminimizing turbulence both within and outside of the boundary layer ofairflow. A point of novelty of the current invention lies in fashioninginterfering body 6 with sloping (that is, non-parallel, or converging)sides so as to deliberately encourage turbulent airflow within sensor 2.This design feature results in an area of turbulent airflow within theboundary layer at the base of interfering body 6. As water precipitationis enhanced in turbulent (as opposed to laminar) boundary layers,droplets of water tend to precipitate at the base of interfering body 6,rather than in pressure-sensing circular lumen 16 (which is distant fromthe base of interfering body 6), thus preventing blockage of circularlumen 16 by water droplets formed by condensation. Although in apreferred embodiment of the current invention the surfaces ofinterfering body 6 are concave, it is envisaged that the surfaces ofinterfering body 6 may be of any shape, including being flat, withoutdeparting from the spirit of the current invention.

Sensor 2 functions as a differential-pressure flow sensor in anidentical manner to that described for standard differential-pressureflow sensors, whereby static and dynamic pressures are sensed at edgeportions 8 and 10 at either end of interfering body 6. It should benoted, however, that in terms of the functioning of the currentinvention, the actual points at which pressures are measured, using theventuri principle, are at circular lumens 16 and 18, which are distantfrom the inner surface of cylindrical body 4. Thus, accumulation ofwater on the inner surface of cylindrical body 4 does not causeobstruction of circular lumens 16 and 18, even if movement of thepatient or of the respiratory tubing causes the accumulated water toflow within cylindrical body 4. It will be understood that circularlumens 16 and 18 on horizontal shelves 40 and 42 may be positioned atessentially any vertical displacement along edge portions 8 and 10 whichwould allow water to accumulate on the inner surface of cylindrical body4 without obscuring circular lumens 16 and 18, without departing fromthe spirit of the current invention.

FIG. 5 illustrates the pattern of airflow generated by edge portion 8.The inclination of edge portion 8 generates an airflow vector (indicatedby the arrows marked “C”) that is directed towards circular lumen 18.This is in contrast to existing differential-pressure flow sensors, inwhich the leading edge portion is vertical and is not designed tospecifically create a flow vector directed at the oppositepressure-sensing port. In the current invention, airflow along thisvector has the effect of “flushing” water droplets out of circular lumen18. It will be understood that edge portions 8 and 10 may be fashionedin essentially any manner that produces an airflow vector directedtowards circular lumens 18 and 16 respectively, without departing fromthe spirit of the current invention.

In a preferred embodiment, as shown in FIG. 6, cylindrical body 4 isfashioned in a Y-type configuration, such that interfering body 6 ispositioned at the junction of the limbs of the Y, each of the twoproximal limbs being functional to convey either inspiratory orexpiratory airflow only, and the single distal limb being functional toconvey bi-directional airflow to and from the patient interface. In analternative embodiment, as shown in FIG. 1, cylindrical body 4 isexecuted in a straight cylinder configuration.

It should be noted that flow sensor 2 may be located at essentially anypoint along the path of airflow of a patient, whether such point bewithin the patient's respiratory tract, within the tubing of aventilator connected to a patient, within a ventilator itself, or withinessentially any device which receives either positive-pressure ornegative-pressure airflow from a patient. In particular, as shown inFIG. 7, flow sensor 2 may be located at the distal end of anendotracheal tube 106. Similarly, it is envisaged that flow sensor 2 maybe located in a tracheostomy cannula, a suction catheter, abronchoscope, or any other instrument that may be inserted into therespiratory tract.

FIG. 8 illustrates an alternative embodiment of interfering body 6,wherein second length 36 is not recessed in relation to first length 20.In this embodiment, circular lumen 16 opens into a recess 108 withinsecond length 36 (rather than opening onto shelf 40 as in the preferredembodiment of interfering body 6 illustrated in FIGS. 1 through 6). Assuch, circular lumen 16 is located within interfering body 6, ratherthan on edge portion 8 of interfering body 6. Thus in both embodimentscircular lumen 16 is located in edge portion 8: either recessed withinedge portion 8 or disposed upon edge portion 8.

Although the current invention has been described as a ventilatoryairflow sensor, it is envisaged that the current invention may be usedin any application, both within and without the field of medicine,wherein the measurement of humid gas flow may be desirable.

There has therefore been described a differential-pressure flow sensorthat, without the use of electrical components in the sensor, preventscondensation of water vapor in the pressure-sensing lumens of the sensorand prevents blockage of the pressure-sensing lumens by condensed waterwithin the respiratory tubing.

While the present invention has been described with reference to one ormore specific embodiments, the description is intended to beillustrative of the invention as a whole and is not to be construed aslimiting the invention to the embodiments shown. It is appreciated thatvarious modifications may occur to those skilled in the art that, whilenot specifically shown herein, are nevertheless within the true spiritand scope of the invention.

1. A respiratory tract differential-pressure flow sensor, comprising: atube having a distal end that is insertable into a respiratory tract; aninterfering body disposed within said tube proximal to said distal endand having a first edge facing one end of said tube and a second edgefacing away from said end, said interfering body extending across thediameter of said tube; a first pressure sensing port operative to sensean air pressure, said first port being disposed in said first edge notabutting the wall of said tube; and a second pressure sensing portoperative to sense an air pressure, said second port being disposed insaid second edge not abutting the wall of said tube, wherein either ofsaid edges is inclined inwardly towards the axis of said interferingbody extending across said diameter of said tube.